Abstract:Changes in hydrological conditions are expected and may alter carbon cycling in peatlands. Peat aeration with water table change has not commonly been investigated and the water table is often assumed to constitute the oxic-anoxic boundary in peat. We analyzed temperature, moisture, oxygen (O2) and carbon dioxide (CO2) concentrations in profiles of a temperate fen during two seasons. A drying-rewetting cycle and flooding were induced and compared to controls. The response of moisture and water table position varied greatly and was related to gradients of peat compaction and ash content. Background drought raised air filled porosity (AFP) to a maximum of 15 - 38% in shallow peat and experimental drought up to 50%. Decline in water table and soil moisture broadly led to O2 penetration and CO2 degassing and rewetting and flooding to anoxic conditions and CO2 accumulation in peat pore water. In dense peat with {greater than or equal to} 20% ash content the unsaturated zone remained partly low in oxygen, however, and up to 5% AFP and 20 cm above water table O2 concentrations frequently remained below 50 µmol L-1. Moderately intense and short drying did not induce substantial oxygen penetration in the compacted soil profiles. The likelihood of presence of oxygen in the peat was predicted by logistic regression using water table and ash content or bulk density as predictors (p < 0.0005). The model is potentially useful for predicting the position of the redoxcline in peat deposits and may assist in improving statistical models of trace gas emission from peatlands.